An optical recording medium, rotationally driven at a constant angular velocity (CAV), has a physical format as shown in FIG. 1, in which pits 71, each having a pit width of 0.5 .mu.m and a pitch length of 0.86 .mu.m, are formed at a track pitch of 1.6 .mu.m in the radial direction, and in which the pits 72 are in an ordered array in a direction extending along the recording track.
These dimensions are based on manufacture constraints and on the diameter of a beam spot 71 on the optical recording surface of a light beam outgoing from a playback laser diode and converged by an objective lens.
For improving the recording density of the above-mentioned optical disc, it may be contemplated to diminish the track pitch, pit length and the spot size of the light beam on the recording surface.
In the case of e.g. a magneto-optical disc, constraints are imposed that pits recorded on the basis of a given accessing unit be independent from the pits recorded on the basis of other accessing units, and that all data based on the accessing units have to be rewritten. However, for achieving high density, the spot scanning rate is lowered, with the spot size of the light beam remaining unchanged, and a pit next to be formed is formed in superimposition on a part of the previously formed pit by way of overwriting for effectively reducing the pit length. That is, the recording density is increased in the track direction for achieving a magneto-optical disc having a recording density higher than that of a so-called compact disc as a read-only optical disc having the same diameter as the magneto-optical disc.
It may also be envisaged to narrow the track pitch for increasing the recording density in the radial direction of the optical disc. If the spot size on the recording surface of the light beam should remain unchanged, a light beam is radiated on pits of a neighboring recording track. The result is a cross-talk and a lower S/N ratio and, in the worst case, data can not be reproduced.
It may be contemplated to narrow the track pitch and to reduce the spot size of the light beam on the recording surface simultaneously. However, since the spot size of the light beam on the recording surface is proportionate to the wavelength of the light beam and inversely proportionate to the numerical aperture NA of an objective lens, it becomes necessary to develop a laser light source of a shorter wavelength or a large-sized expensive lens with a view to increasing the numerical aperture NA.
It may also be envisaged to keep the spot size of the light beam unchanged as before and to narrow the track pitch as well as to narrow the pit width to inhibit cross-talk to raise the recording density. However, if the pit width is reduced, the production yield of the optical disc tends to be lowered, while it becomes impossible to employ a customary cutting device.
Besides, when recording of reproducing the information on and from the optical recording medium, such as an optical disc, it becomes necessary to effect tracking control to cause the light beam to scan the track center at all times.
For tracking control, there is known a sample servo system in addition to a continuous servo in which tracking control is performed using a pre-groove formed on a disc substrate of the optical disc. Tracking Control in the sample servo system is effected with the aid of a pair of wobble pits 51a, 51b preformed on the optical disc with a shift of one-fourth of a track pitch in mutually opposite directions from the center of the recording track formed on the disc, that is the track center, as shown in FIG. 2.
Specifically, a tracking error signal is found based on a difference between the signals derived from sampling of the amount of the reflected light produced when the spot 53 of the light beam traverses the wobble pits 51a, 51b. Tracking control is effected by radially shifting the spot 53 so that the tracking error signal becomes equal to "0".
On the other hand, track jump for shifting the light beam spot to a neighboring track or a separate recording track for recording the information or reproducing the information on or from such neighboring or separate track is performed in the following manner. First, a tracking control loop is once opened and the light spot is moved to the vicinity of the target track. The tracking control loop is again closed to effect tracking control again to shift the spot to the target track.
During the above-mentioned track jump, that is when the light beam spot is scanned obliquely across the recording track of the optical disc, the tracking error signal in the sample servo system becomes a sine wave signal as shown in FIG. 3 so that the tracking error signal is not defined monistically with respect to the displacement x of the light spot from the center of the recording track. It is within a range 61 indicated by hatching lines in FIG. 3 that the tracking error signal is defined monistically with respect to the displacement x. That is, it is only when the light spot is within the range 61 shown in FIG. 3 that the light beam spot may be moved stably to the center of the recording track.
On the other hand, if the displacement x is larger so that the beam spot is outside the range 61 shown in FIG. 3, tracking control becomes unstable or in a state of oscillation. Such situation tends to be incurred in case of a larger relative movement velocity of the light beam spot along the radial direction of the disc. If there is an error in a distance by which the light beam spot has been moved on opening the tracking control loop during track jump, and the tracking control loop is closed outside the range 61 shown in FIG. 3, there is a risk that the light beam spot be moved to a recording track other than the target recording track. In such case, it becomes necessary to effect track jump a second time. However, such track jump cannot be undertaken until stabilization of the track control loop.
Thus the conventional tracking control system has a drawback that track jump cannot be achieve in stability.
Besides, if the track pitch is narrowed as described above for raising the recording density of the optical disc, it becomes necessary to provide wobble pits 51a, 51b offset by a distance equal to a quarter track along the radius of the disc, in the case of the sample servo system, as shown in FIG. 2, so that it has not been possible to narrow the track pitch.
In view of the above-described status of the art, it is an object of the present invention to provide an optical recording medium capable of high density recording without changing the pit width, pit length or the size of the light beam spot, and a method for recording or reproducing the information on or from such recording medium.
It is another object of the present invention to provide a recording method for an optical recording medium capable of increasing the recording density as compared to that achieved with the conventional recording method, an optical recording medium capable of high density recording, and a method for generating tracking error signals capable of achieving stabilized tracking control.